1. Field of the Invention
The present invention relates to a pickup device for recording or reproducing information to or from an optical information recording medium, such as an optical disc, recorded with information on the track. More particularly, the invention relates to a buffer pad attached to an objective lens or a driving part used in the pickup device.
2. Description of the Related Art
In recent years, the optical discs of the optical information recording media are broadly used as means for recording and reproducing data such as images and sound. In a recording/reproducing apparatus, such information is recorded or reproduced to or from a track of the optical disc by the pickup device relatively moving thereto. To the pickup device, there is a demand for further increase of density and reduction of size. In the objective lens of the pickup device, the numerical aperture for example is given 0.45 for the CD (Compact Disc) and 0.6 for the DVD (Digital Versatile Disc). Furthermore, the effective diameter of the objective lens has decreased from nearly 4.5 mm in the conventional to nearly 3 mm. As an influence of size reduction for the pickup device, so-called the working distance of between the tip end of the objective lens and the surface of the optical disc has been decreased. As compared with the conventional structure, there is increased possibility of collision of the objective lens with the optical disc surface.
Furthermore, the thickness of the transmissive substrate, while 1.2 mm for the CD, is 0.6 mm for the DVD. Because the distance is shortened to the recording surface protected by the transmissive substrate, there is increased influence upon the signal based on the injury on the optical disc surface caused by collision of the objective lens.
Meanwhile, recently the advent of the DVD-RW (Digital Versatile Disc-Rewritable) standard has allowed the user to make record onto the optical disc. During data recording by the user, the presence of injury on the disc surface might cause a serious affection.
For a recording/reproducing apparatus for recording/reproducing information on an optical disc loaded therein, a focus servo and a tracking servo are essential for always accurately converging light beams for writing and reading information to a pit train or the like formed spirally or concentrically on a recording surface of the optical disc. The focus servo performs a positional control for an objective lens, used to irradiate a pit train on the optical disc with light beams, in an optical axis direction so as to reduce a focus error, i.e., an error of the position of the objective lens in the optical axis direction with respect to the focus position of the objective lens. The tracking servo performs a positional control for the position of the objective lens, used to irradiate a pit train on the optical disc with light beams, with respect to a recording track in a radial direction of the optical disc, so as to reduce a tracking error, i.e., an error of the objective lens with respect to the pit train recording track position.
FIG. 1 illustrates a conventional optical pickup device using an astigmatism method. A laser beam from a semiconductor laser 1 is transformed into a parallel laser beam by a collimator lens 2, passes through a polarizing beam splitter 3 and a xc2xc wavelength plate 18, and is converged by an objective lens 4 toward an optical disc 5 to form a light spot onto a pit train on an information recording surface of the optical disc 5.
Light reflected from the optical disc 5 is converged by the objective lens 4 and directed by a beam splitter 3 to a detecting lens 7. Converged light formed by the detecting lens 7 passes through an astigmatism generating element 8 such as a cylindrical lens, multi lens and the like, to form a spot near the center xe2x80x98Oxe2x80x99 of a light receiving surface of a quadrant photodetector 9 having four light-receiving surface areas (elements) divided by two orthogonal line segments. The multi lens 8 irradiates the quadrant photodetector 9 with a light spot SP in the shape of true circle as illustrated in FIG. 2A when the laser beam is converged on the recording surface of the optical disc 5 in focus, and an elliptic light spot SP, extending in an orthogonal direction of the elements as illustrated in FIG. 2B or 2C when the converged laser beam is out of focus on the recording surface of the optical disc 5 (FIG. 2B illustrates the light spot SP when the objective lens 4 is too far from the optical disc 5, while FIG. 2C illustrates the light spot SP when the objective lens 4 is too near the optical disc 5 shown in FIG. 1), thus generating so-called astigmatism.
The quadrant photodetector 9 opto-electrically transduces the light spot irradiated to the four light receiving surface areas into respective electric signals which are supplied to a focus error detecting circuit 12. The focus error detecting circuit 12 generates a focus error signal (FES) based on the electric signals supplied from the quadrant photodetector 9 and supplies the focus error signal to an actuator driver circuit 13. The actuator driver circuit 13 supplies a focusing driving signal to an actuator 15. The actuator 15 drives the objective lens 4 in response to the focusing driving signal in the optical axis direction.
The focus error detecting circuit 12, as illustrated in FIG. 3, is connected to the quadrant photodetector 9, where the quadrant photodetector 9 is composed of four detecting elements DET1 to DET4 in first to fourth quadrants which are located adjacent to each other with two orthogonal division lines L1 and L2 interposed therebetween and which are independent of each other. The quadrant photodetector 9 is positioned such that the division line L2 is in parallel with a tangential direction with respect to the extending direction of the recording track, and the other division line L1 is in parallel with the radial direction of the same. Respective opto-electrically transduced outputs from the elements DET1 and DET3, symmetric with respect to the center xe2x80x98Oxe2x80x99 of the light receiving surface of the quadrant photodetector 9, are added by an adder 22, while respective opto-electrically transduced outputs from the elements DET2 and DET4, also symmetric with respect to the center xe2x80x98Oxe2x80x99 of the light receiving surface, are added by an adder 21, and outputs from the respective adders 21 and 22 are supplied to a differential amplifier 23. The differential amplifier 23 calculates the difference between the supplied signals, and outputs a signal indicative of the difference therebetween as a focus error signal (FES).
In the focus error detecting circuit 12, the outputs of the quadrant photodetector 9 are added by the adders 21 and 22, respectively, and the differential amplifier 23 calculates the difference between the outputs of the adders 21 and 22 to generate a focus error component. In this event, when the light beam is in focus, the light spot in the shape of true circle as illustrated in FIG. 2A is formed on the quadrant photodetector 9, where a spot intensity distribution is symmetric with respect to the center xe2x80x98Oxe2x80x99 of the light receiving surface of the quadrant photodetector 9, i.e., symmetric in the tangential direction and in the radial direction, so that the values resulting from the additions of the opto-electrically transduced outputs from the elements on the diagonals are equal to each other, with the focus error component being calculated to be xe2x80x9czeroxe2x80x9d. On the other hand, when the light beam is out of focus, i.e., an elliptic light spot extending in a diagonal direction as illustrated in FIG. 2B or 2C is formed on the quadrant photodetector 9, so that the values resulting from the additions of the opto-electrically transduced outputs from the elements on the diagonals are different from each other. Thus, the focus error component output from the differential amplifier 23 exhibits a value corresponding to the focus error. Specifically, assuming that the references designated to the elements of the quadrant photodetector 9 represent the outputs thereof, the focus error signal FES is expressed by the following equation:
FES=(DET1+DET3)xe2x88x92(DET2+DET4).
Where the optical disc is injured on the surface or the optical disc has fine bubbles in a transparent cover layer thereof, the light spot on the photodetector is disturbed in shape by the influence thereof. As shown FIG. 4, if the light spot SP on the light-receiving surface of the quadrant photodetector 9 is disturbed in shape despite of a proper distance of the objective lens from the optical disc, then a pseudo focus error signal occurs having the same polarity as that of the optical disc in far distance in focus servo system. The pseudo focus error signal causes current flow to a driving mechanism for the objective-lens of a drive part in the pickup device. As a result, the objective lens may be driven forcibly toward the optical disc. Due to this, a pseudo error signal is incidentally generated great or the objective lens cannot be returned to a correct position even after passing an injured region due to overlap with the vibration of the pickup device and optical disc, resulting in collision between the objective lens 4 and the optical disc 5. If the injury is in an initial stage and light in extent, an instantaneous focus error signal occurs but servo operation is recovered immediately after passing the injury. However, if injury is deep, reading would be impossible.
In the conventional CD reproducing device, because the numerical aperture NA of the objective lens is small with great focus depth, a certain degree of noise over the focus error signal (FES) will not cause a problem of focus error. However, where reading information out of an optical disc such as a DVD-RW, a working distance is short and the objective-lens numerical aperture is great with small focus depth. Accordingly, there is increased effect of the noise contained in the focus error signal upon focus servo.
Due to this, particularly in the situation of causing injury to the optical-disc surface, the objective lens is contacted with the optical-disc surface repeatedly in a particular track position. With such contact, the surface area the light beam passes is injured when the objective lens traces the track area, inducing further contact occurrence. This results in a problem that signal recording or reproducing to or from the track is difficult.
The present invention has been made in view of the foregoing points, and it is an object to provide a pickup device having a collision preventive function capable of suppressing the spread of damage caused due to collision between the objective lens and the optical-disc surface.
A device according to the present invention is a pickup device in an apparatus for performing recording or reproducing, with relative movement, to or from an optical information recording medium recorded with information on a track, which comprises:
an objective lens for focusing a light beam to a track on a recording surface of an optical information recording medium;
a drive part supporting the objective lens to control a position thereof relatively to the optical information recording medium; and
a collision preventive device provided on the objective lens or drive part to prevent direct contact between the objective lens and a surface of the optical information recording medium; wherein
the collision preventive device includes a contact portion to be contacted with the surface of the optical information recording medium earlier than the objective lens when the objective lens approaches the surface of the optical information recording medium, and a non-contact portion provided adjacent the contact portion not to be contacted with the surface of the optical information recording medium in an area corresponding to a path of a light spot defined by a diameter of the light spot of a light beam on the track.
In one aspect of the pickup device according to the present invention, said optical information recording medium is an optical disc recorded with information concerning said optical information recording medium in a lead-in or lead-out region existing in an inner periphery or outer periphery thereof, said contact portion being positioned on an inner or outer side of said lead-in or lead-out region of said optical disc when said objective lens focuses a light beam to said lead-in or lead-out region.
In the invention, because the collision preventing device, i.e., buffer pad is provided in the vicinity of the objective lens to structure a protection pad for contact avoiding the track area to be read, in which the non-contact portion of the buffer pad is not contacted with an area of the surface of the disc within an effective diameter of the light spot on the track (in this paper, such area is also referred merely to as xe2x80x9ceffective diameterxe2x80x9d) under recording or reproducing. Thus, it is possible to prevent against a chain of spread of collision damage due to the objective lens. Also, because in the innermost periphery of the optical disc the optical-disc information region is structurally avoided in contact, prevention is made against a chain of spread of collision damage due to the objective lens.